This kind of problem (not particularly related to campers) is not at all uncommon.
They are only in title to have one go fixing it which they haven’t done.
The remedy is quite simple.
Go to someone now who is competent, get it fixed, and then take the incompetents to court for that cost.
www.moneyclaim.gov.uk is quick and easy.
I would suggest that you tell them that’s what you are going to do.
Because you don’t want to go back to the original repairer, that really is your only option.
Your claim is against the retailer and it’s their fault if they used an incompetent repairer.
If I have answered your question for you, at the top of the page, you should see a rating facility. Can you please rate my reply 3 stars or more because that really helps me? Please note you are not rating whether it’s favourable, because sometimes you may not get the answer you want, but basically my knowledge and way of dealing.
The thread does not close and I am happy to answer any questions you may have arising from this.